Effectiveness of pre- and postharvest silicon and phosphorous acid applications in inhibiting Penicillium digitatum on citrus fruit.
Citrus is, by tonnage, globally the most-produced fruit. Although technological advances have greatly improved storage life and quality of citrus, postharvest decay remains a major problem. Penicillium digitatum (green mold) and P. italicum (blue mold) are the most economically important postharvest pathogens. Over the years, fungicides belonging to the benzimidazole, thiabendazole (TBZ), benomyl, and imidazol (IMZ) groups have been used extensively to control these diseases; however, the development of fungicide-resistant strains of the pathogens together with the withdrawal of effective chemicals from the market has led to the search for more integrated methods of disease control. Silicon and phosphorus are able to trigger some ‘systemic responses’ that enhances fruit resistance to pathogen attack. The aim of this study was to ascertain the changes in biochemical composition of fruit after application of these two chemicals with the intention to improve the current understanding of their mechanisms of action. A proper understanding of these mechanisms could allow for the manipulation of fruit metabolism to improve the level of disease control. Two orange cultivars (‘Delta’ Valencia and ‘Washington’ navel) as well as one lemon cultivar (‘Eureka’) from Ukulinga Research Farm, Pietermaritzburg, were used. Fruit were treated both, pre- and postharvest with three different concentrations of potassium silicate (Si, 1250mg ℓ-1, 2675mg ℓ-1 and 5350mg ℓ-1) and one concentration of phosphorous acid (P, 500mg ℓ-1) as well as a combination of each of the Si concentrations together with P (1250 + 500, 2675 + 500 and 5350+ 500 mg ℓ-1). For the pre-harvest experiment, trees bearing fruit were treated by a soil drench around the base of the trunk with 5 ℓ treatment solution. This treatment was carried out for four consecutive weeks leading up to harvest. As a postharvest treatment, fruit were immersed in treatment solutions for a period of 90 s. Control fruit as well as control trees were treated with water. Following these applications, fruit were inoculated with a 1 x 104 mℓ spore suspension of P. digitatum, stored at 5.5oC, and sampled for 4 biochemical analysis ten days later. Petroleum jelly was applied over the area where the peel tissue had been sampled in order to prevent fruit desiccation and allow for disease monitoring. Disease lesion size (mm), as well as total rind phenolic and flavonoid concentrations were determined. Results were compared by analysis of variance (ANOVA) followed by Fisher’s Protected Least Significant Difference test (P< 0.05) using GenStat® Version 14. When applied at the lowest concentration (1250 mg ℓ-1), the Si treatment provided the most effective disease inhibition, as these fruit developed the smallest average lesion size. The two higher Si concentrations (2675mg ℓ-1 and 5350mg ℓ-1) were not significantly different (P>0.05) from each other and the control. Phosphorous acid provided less disease control than all other treatments and the control. Although the treatment combinations did not have a synergistic effect on disease suppression, they delayed disease onset and sporulation compared with the treatments alone. There were significant differences in the level of disease inhibition achieved by the treatments, but differences in phenolic and flavonoid concentration between treatments were not consistently significant; it can, however, be concluded that there was a correlation between disease control and increased rind phenolics. Increasing the concentration of Si did neither result in a significant increase in the level of disease control, nor in an increased production of rind phenolics or flavonoids. Separate Si and P treatments proved to be more effective in hindering disease spread than the combination of these treatments. Further research is required to fully understand the biochemical changes that these chemicals induce and to determine which mode of action they follow.